Rampant Killer Unicorns Series, Book 1.

Forget everything you ever knew about unicorns. The fluffy, sparkly, friendly "horses" so popular the world over don't exist. Real unicorns are killers. Beasts the size of elephants, with cloven hooves that shake the earth, hides impervious to bullets, and horns that contain a deadly poison, unicorns can outrun a sports car and smell a human from a mile away. And they can only be killed by virgin warriors descended from Alexander the Great....